And I will echo what M0 said, you have a DC ground, not a good RF ground, they are two different things entirely.
You can run a 16 gauge piece of wire from your mount to the frame and show low ohms to the rest of the truck at DC but RF will not see it that way.
This is true shizzle, you can take a field strength meter and see that you are getting "some" gain out the front, but not like you would if you properly mounted the antennas over a nice solid sheet of metal.
Even the guys on the other forum you have been asking the same question too have been telling you the same thing.
If Peek-A-Boo is giving you advice about your antenna set up it would be in your best interest to listen to him..... He has played this game before.
You do not get trophies at shootouts running lossy antenna set-ups.
The suggestion he gave you about mounting a camper shell and lining the inside with metal is a damn good one.
I have some pictures somewhere of a little pickup with a camper shell that is lined with copper mesh and bonded to the bed and frame.
It was a kick ass set-up......And I do mean kick ass.
73
Jeff